This invention relates to novel powders comprising coatings of an electrically conductive silicide, carbide, or boride on the surface thereof.
There has been a great deal of inventive effort devoted to producing small, electrically-conductive metal powders for use in electromagnetic energy shielding applications. One of the primary problems to be overcome was to develop a substitute for expensive silver loadings. To this end a number of substitutes were suggested, e.g. silver-coated copper and silver-coated glass. The former material has proved to be the choice in most applications because of its lower cost and high metal content. Nevertheless, because copper tends to migrate into and through the silver powder, eventually forming an oxide of high electrical resistivity, work continues in an attempt to provide a suitable alternative to the favored approach.
In a different field of technology other investigators have been working to provide improved ferromagnetic powders for use in magnetic recording media. A great deal of work has been done in an attempt to provide particles of good corrosion resistance and of high magnetic moment. Iron, a particularly desirable candidate for such applications, is not favored because of its susceptibility to corrosion. Therefore most magnetic recording members have been made out of iron oxides. Such relatively exotic materials as "chromium dioxide" and high-cobalt alloys also have been developed for use in magnetic applications. However, where iron itself has been utilized, its potential effectiveness is grossly reduced because of the need to use an extraordinary amount of chemical stabilizers in the formation. Some patents generally descriptive of the work being done in this magnetic recording field include U.S. Pat. Nos. 3,649,541; 3,810,840; 3,586,630; 3,740,266; 3,149,995; 3,650,828; 3,630,771; 3,597,273 and many others. Among other patents relating to this microwave shielding application and, especially, fillers useful therein are U.S. Pats. Nos. 3,140,342; 3,202,488; 3,476,530; 3,583,930; 3,609,104; 3,620,873 and 3,648,937.
Materials of improved properties and sought for use not only in the fields described above but also in making conductive formulations for use as flowable "solders", e.g. epoxy solders filled with silver, and the like. The achievement of providing a relatively inexpensive, chemically inert, powder of suitable electrical conductivity or suitable ferromagnetic character has eluded investigators. Gold and silver are still used when excellent chemical resistance or chemical stability are required.
In a hindsight search of prior work, a search made in view of the invention disclosed in this application, it was noted that carbide powders have been used as superconductive (U.S. Pat. No. 3,723,359), as a conductor in a ceramic material, as non-contacting, yet conductive, particles in a matrix to form a lossy dielectric material. None of these applications suggest the use of carbides or like materials as protective coatings which utilize the morphology of the coating to (1) protect the particulate mass or substrate and (2) to preserve the conductivity of the composition as a whole.
U.S. Pat. No. 3,671,275 to Gates wherein reflection of microwaves is said to be experienced at the expense of absorbing energy when conductive particles are in particle-to-particle contact. Gates relies on relatively small eddy current losses in large SiC particles to absorb energy. Such effects are relatively small when compared to the energy absorption achievable with the magnetic and conductive powders described hereinbelow.
Also, it is noted that nitrided metal particles are described in U.S. Pat. No. 3,094,448 to Takahashi et al. The application teaches that iron nitride protects a high-iron alloy from corrosion when the coated particles are placed in magnetic tape. Takahashi does not say what nitride he uses, but Fe.sub.2 N is water soluble and probably undesirable for use in Takahashi's process. Moreoever, Fe.sub.3 N and Fe.sub.4 N are inferior in electro-conductivity to the carbides and the like as described below.
Also in hindsight it is noted it has been suggested in U.S. Pat. No. 2,958,936 to use carbides as insulating materials in forming highly resistive so-called "capillary-conducting particles". The author appears to have little or no interest in electrically-conductive materials.
It is to be emphasized that the primary use of carbides and silicides according to the invention is to protect the surface characteristics of materials, particularly. Even if a metal is highly corrosion-resistant because of its ability to form a non-conductive protective oxide or sulfide on the surface thereof, it is a metal that can be suitably treated according to this invention and which will, upon treatment, be vastly improved in its long-term electroconductive capacity. Thus, in concept, the invention differs from such prior art as disclosed by Takahashi which utilizes nitrides for the purpose of protecting the entire metal powder, e.g. one formed mostly of iron, from corrosion.
There has been at least one previous manufacture of a carbide-coated metal powder. U.S. Pat. No. 3,901,689 to Pelton describes the manufacture of a chromium-carbide-coated chromium powder and the use of this powder in metallurgical compositions to facilitate the wear properties thereof. However, as might be inferred from the intended use, the carbide coatings of Pelton's materials are too thick to provide a good electro-conductive powder using a relatively-high-resistant carbide like chromium carbide.